The principal problem under investigation in this laboratory centers on changes in the external cell surface which correlate with increased cellular age in culture. Research goals during fiscal year 3 (1977-1978) of this program will focus on: (a) Further investigation of the variation and distribution of lectin binding sites on surfaces of early and late passage cells utilizing scanning electron microscopy, transmission electron microscopy, Nomarski interference optics and freeze-fracture microtomy. (b) Investigation of the potential decrease in metabolic coupling between aging cells as suggested by the paucity of communicating junctions (i.e. focal tight and gap junctions) in aging cultures using mutant lines as vehicles for quantitation. (c) Continuation of studies exploring alterations in age-associated cell behavior which may have a basis in the differentiation of the cell surface (viz. kinetics of adhesion to a substrate under conditions of reduced surface glycosaminoglycan composition). BIBLIOGRAPHIC REFERENCES: Kelley, R.O. Development of the aging cell surface: a freeze-fracture analysis of gap junctions between human embryo fibroblasts aging in culture. Mech. Aging Devel. 5: 339-345, 1976. Kelley, R.O. and J.F. Fallon. An ultrastructural analysis of the apical ectodermal ridge during vertebrate limb morphogenesis. I. The human upper limb with special reference to differentiation of cell junctions and extracellular laminae. Devel. Biol. 51: 241-256, 1976.